Bacteria can assimilate many carbon sources, and various specific systems exist for their cellular transmembrane transport. Moreover, most of bacteria can respond to environmental changes to survive under a limited nutritious condition. Their cells are provided with a detector for monitoring the environment to select their nutrition from various carbon sources. Examples of such transmembrane transport systems and detectors of sugars include PTS (phosphoenolpyruvate/carbohydrate phosphotransferase system or phosphoenolpyruvate-sugar transport system; as for PTS, refer to Escherichia coli and Salmonella Cellular and Molecular Biology, Second Edition, ASM (American Society for Microbiology) Press).
PTS is involved in regulation of transmembrane transport and phosphorylation of various sugars (PTS sugars), movement towards these carbon sources and many metabolic pathways. PTS catalyzes the following reaction. PEP refers to phosphoenolpyruvic acid.PEP (intracellular)+Sugar (extracellular)-->Pyruvic acid (intracellular)+Phosphorylated sugar (intracellular) 
PTS catalyzes a reaction for generating a phosphorylated sugar and pyruvic acid by translocating a phosphate group of intracellular phosphoenolpyruvic acid (also referred to as “PEP” hereafter) to an extracellular sugar. The phosphorylation of a sugar is linked with cellular transmembrane transport of a sugar, and energies required for these processes are supplied from PEP, which is an intermediate of the glycolytic pathway.
In Escherichia coli and Salmonella typhimurium, proteins constituting PTS catalyze the following reactions.PEP+EI-->P-EI+Pyruvic acid  (1) P-EI+Hpr-->P-Hpr+EI  (2) P-Hpr+EIIA-->P-EIIA+Hpr  (3) P-EIIA+EIIB-->P-EIIB+EIIA  (4) P-EIIB+Sugar (extracellular)+EIIB+Sugar-P (intracellular)  (5) 
Among proteins involved in the above reactions, EI (Enzyme I) and Hpr (histidine protein) are soluble cytoplasmic proteins involved in phosphorylation of all PTS sugars and referred to as general PTS proteins.
On the other hand, EII (Enzyme II) is specific for PTS sugars and consists of several domains or proteins depending on the sugars. For example, the mannitol-specific EII is a membrane-bound protein consisting of three domains, A, B and C. The glucose-specific EII and sucrose-specific EII consist of IIB and IIC, which are membrane-bound proteins, and IIA, which is a soluble protein. In any case, translocation of a phosphate group from PEP to a sugar is mediated by EI, HPr, EIIA and EIIB. The EIIC domain, which is an intramembranous portion of EII, forms a translocation channel and is considered to be probably a specific binding site of a substrate.
The third type of EII is observed in mannose PTS. Both of its domains A and B are fused in a single soluble polypeptide, and the two intramembranous proteins (IIC and IID) are involved in transmembrane transport of mannose.
In Escherichia coli and Salmonella typhimurium, the gene encoding EI (ptsI) has been cloned and sequenced (Saffen, E. W. et al., J. Biol. Chem., 262, pp.16241-16253, 1987: De Reuse, H. and Danchin, A., J. Bacteriol., 170, pp.3827-3837, 1988). Further, EII specific for some sugars have also been cloned (Saffen, E. W. et al., J. Biol. Chem., 262, pp.16241-16253, 1987; Erni, B. and Zanolari, B., J. Biol. Chem., 261, pp.16398-16403, 1986; Nelson, S. O. et al., EMBO J., 3, pp.1587-1593, 1984).
It is known that some kinds of sugars are taken up by non-PTS, which do not require PEP, as a system for uptake into cells.